Correction of errors caused by ambient non-uniformities in a fringe-projection autofocus system in absence of a reference mirror

ABSTRACT

Fringe-projection autofocus system devoid of a reference mirror. Contributions to error in determination of a target surface profile caused by air non-uniformities measured based on multiple measurements of the target surface performed at different wavelengths, and/or angles of incidence, and/or grating pitches and subtracted from the measured profile, rendering the system substantially insensitive to presence of air turbulence. Same optical beams forming a fringe irradiance pattern on target surface are used for measurement of the surface profile and reduction of measurement error by the amount attributed to air turbulence.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application claims benefit of and priority from the U.S.Provisional Patent Application No. 61/875,967 filed on Sep. 10, 2013 andtitled “Correction for Errors Caused by Air Turbulence in aFringe-Projection Autofocus System in Absence of a Reference Mirror”.The disclosure of the above-identified provisional patent application isincorporated herein by reference.

TECHNICAL FIELD

The present invention relates generally to an optical autofocus systemfor a lithographic exposure tool and, in particular, to an autofocussystem employing a fringe-projection system causing an output that isindicative of a spatial displacement of a surface of a target of theautofocus system.

BACKGROUND

Non-uniformities of optical paths such as those caused by heterogeneityof the ambient environment—for example, air turbulence due to airmovement or non-uniformities of the refractive index—continue to presenta practical problem for operation of an optical autofocus (AF) system(unless such a system is completely sealed off from the environment,which is costly and not always practically feasible for operation). Thepresence of air turbulence, for example, manifests in changes in thephase acquired by light propagating through the fringe-projection systemof an optical autofocus system on its way to the detector, and,therefore, substantially un-quantifiably contributes to the error in themeasurement of a target surface profile. As the degree of accuracy andprecision of the operation of the AF systems continues to improve, thereremains an unmet need in an operational modality that enables toidentify the air-turbulence related measurement error, controllably andwith a known degree of certainty.

SUMMARY OF THE INVENTION

Embodiments of the invention provide a system that includes aprofile-generating element configured to generate a profile of a targetsurface from optical data derived from two or more selected opticalparameters associated with an irradiance pattern formed on the targetsurface, the optical parameters selected to be indicative of the ambientair turbulence around the target surface so that profiling errorsinduced in the profile by the ambient air turbulence are at leastpartially reduced.

Embodiments of the invention provide a lithographic autofocus systemthat includes (i) an optical projection system structured to project anirradiance pattern representing interference fringes onto a targetsurface and characterized by opto-geometrical parameters (in a specificcase, an interference pattern formed by two plane waves), and (ii)data-processing circuitry configured to generate data that represent aprofile of said target surface, in which profile the errors (that areassociated with differences between at least two optical paths definedby the optical projection system and caused by non-uniformity of air,and that are calculated based only on said opto-geometrical parametersand data representing said irradiance pattern) are at least partiallyreduced.

Embodiments of the invention additionally provide a lithographicautofocus system that includes an optical projection system containing adiffraction grating to define interference fringes on a target surfaceby projecting at least two optical beams formed by said diffractiongrating from light incident thereon. The lithographic autofocus systemadditionally includes a detection system that contains data-processingcircuitry and that is disposed (a) to receive images of the interferencefringes at multiple wavelengths and (b) to determine, based on directmeasurement of the received images, phase data that are associated withan optical path length variation (as measured between such at least twooptical beams) caused by non-uniformity of air. The lithographicautofocus system of the embodiments of the invention is devoid of (i.e.,does not contain) a conventionally-used reference mirror.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be more fully understood by referring to thefollowing Detailed Description in conjunction with the generallynot-to-scale Drawings, of which:

FIG. 1 is a diagram schematically presenting a fringe-projectionsub-system of the autofocus-adjusting surface profiling system, whichbenefits from the implementation of an embodiment of the presentinvention;

FIG. 2 is a diagram illustrating a shift of the fringe-pattern, formedby the fringe-projection subsystem, due to the deviation of a targetsurface from its nominal position;

FIG. 3 is a diagram illustrating a concept of air-turbulence-causeddifference in optical lengths corresponding to paths of propagation,through the fringe-projection sub-system of the autofocus-adjustingsystem, of measurement beam corresponding to different orders ofdiffraction formed by the grating of the sub-system;

FIG. 4 is a top view of an embodiment of the sample-holding portion ofthe autofocus-adjusting system of the invention structured toempirically enable an embodiment of the invention;

FIGS. 5A, 5B, and 5C are plots illustrating an implementation of amethod of the invention used to correct the results of the measurementof the target surface profile by the empirically-determined valuerepresenting an air-turbulence-caused error;

FIG. 6 illustrates schematically the spatial divergence of +1 orders ofdiffraction of measurement beams propagating through thefringe-projection sub-system;

FIGS. 7A, 7B, and 7C are plots illustrating general dependencies andtrade-offs of measurement errors as functions of identified parametersof the autofocus-adjusting system;

FIG. 8 is a diagram illustrating schematically a structure of anexposure tool;

FIG. 9 illustrates an example of environment for implementing theinvention according to an embodiment;

FIG. 10 is a flow-chart illustrating an embodiment of a method of theinvention.

DETAILED DESCRIPTION

An autofocus (AF) device or system (also interchangeably referred toherein as an auto-focus-adjusting system) employs one or more sensors tocollect data used to control, mechanically or electromechanically, theadjustment of focus of the optical system or, alternatively the workingdistance at which the identified element of the optical system ispositioned with respect to the target during the operation. Inlithographic equipment, for example, the AF system is often employed tomaintain the identified target surface associated with the semiconductorwafer in focus of the optical system (for example, a projection lens)during the wafer-exposure procedure. Such AF-operation can rely on theuse of a reference beam of light bounced or reflected from a referencesurface or mirror disposed along the target surface that is subject tomeasurement. The optical path(s) that measurement beam(s) of light (usedfor the measurement of the profile of the target surface) and areference beam follow through the system are substantially long, whichis defined by the practical configuration of a typical AF system. Suchoptical paths, therefore, are inevitably subject to measurement errorscaused by non-uniformities of the ambient environment (for example, byslowly moving air) that, in practice, is difficult and costly toeliminate. Even when substantial measures are taken to reduce themagnitude of these errors (for example, through high levels ofenvironmental control), the non-uniformities of the ambient environmentcan still cause errors that are larger than can be tolerated. In thefuture, the allowable level of these errors will continue to decrease.The ubiquity of such errors, the values of which are becoming comparableto the range of required focus correction as the accuracy and precisionof the AF modalities continue to increase, begs a question of whether itis possible to compensate for or negate such measurement errors based onthe measurement itself, without making the costly andoperationally-impractical attempts to reduce the ever-present air flow.

The present invention addresses a problem of assessing a contribution oferrors, caused by non-uniformities of ambient environment, todetermination of a profile of a target surface in an AF system.Conventional fringe-projection systems employing a reference reflectorsimply cannot work perfectly in terms of acquiring ambient-causederrors, because optical paths, through the fringe-projection system, ofbeams interacting with the reference reflector differ substantially fromoptical paths of beams interacting with the target surface, because inan embodiment of the currently used fringe-projection AF system thereference reflectors is substantially displaced from the target surface.Generally, embodiments of the present invention solve this problem bymaking the required assessment based on optical data representing lightthat has interacted with the target surface as part of thefringe-projection operation and (in stark contradistinction with relatedart) without relying on additional reference data conventionally soughtafter and acquired from a reference beam that does not interact with thetarget surface and is redirected to the optical detector by a referencereflector. Because the optical paths charted by the measurement beams ofa system of the present invention are nearly identical and/or equal (asdiscussed below), operational performance of an embodiment structuredaccording to the idea of the present invention in inherently better thanthat of the systems of related art, in that the errors caused by thenon-uniformities of the ambient are substantially reduce and/ornullified. Accordingly, the present embodiments advantageously render areference reflector of an AF system operationally unnecessary, whichsimplifies the structure of the AF system in comparison with AF systemof related art. More specifically, the present solution is achieved byperforming multiple measurements, the respective sensitivities of whichto air non-uniformity (turbulence) are different from one another andbased on which the contribution to the overall measurement error causedby air turbulence or air-index fluctuations is, first, empiricallyquantified and then subtracted from the overall measurement outcome,thereby rendering an AF system substantially insensitive to indexfluctuations of air.

It will be understood from the discussion presented below thatmeasurement errors, caused by air-turbulence or air-non-uniformityeffects and manifesting in erroneous perception of changes in a profileof a target surface, principally differ from errors related touncertainty of such target surface profile. Such distinction stems fromthe fact that the former are subject to local spatial distribution ofair-non-uniformities and generally affect differently the measurementbeams corresponding to light beams corresponding to different orders ofdiffraction upon the propagation through the AF-system, while the latterare common and substantially the same for the light beams correspondingto different orders of diffraction and depend mostly on the geometry ofthe optical train of the system.

The conventional AF-system typically employs a reference beam thatbounces off a so-called reference mirror or reflector that is used inaddition to the measurement beams impinging on the target surface (see,for example, U.S. patent application Ser. No. 13/801,961). Due to thedifference in spatial positioning and/or orientation between thereference mirror and the target surface to be measured, the referencebeam is spatially displaced with respect to the measurement beam(s) inthe proximity of the target surface, and the correlation of dataacquired from the reference and measurement beams is inevitably reducedas a function of such displacement. Moreover, empirical experienceshowed that the identification of the air-turbulence-caused errors inthe measurement of the target surface profile is notoriously difficult,as movement of air and/or air non-uniformity does not follow a normaldistribution.

In stark contradistinction with an approach that, in attempt tocompensate for the air-turbulence-caused errors, modifies theconventional AF-system to make the system sensitivities corresponding tothe measurement beams and the reference beams substantially equal,embodiments of the present invention are directed to making thesesensitivities as different as possible.

The following discussion refers to FIG. 1, which schematicallyillustrates the principle of operation of a fringe-projection sub-system100 employing a diffraction grating 110 and used in an optical autofocussystem. The purpose of the fringe-projection sub-system 100 is toproject (image) a chosen irradiance pattern formed with the use of thegrating 110 onto the target surface 112 of the substrate 114 (such as awafer) and to re-image this pattern from the surface 112 onto the planeof the detector 120.

A simplified optical train of the fringe-projection sub-system includesa fringe-generating portion (or a source portion) 122 that contains amulti-wavelength source of light 124 forming in operation illuminatinglight 125; a grating 110; auxiliary optics such as lenses 126A, 126B;and a spatial filter 128. The optical elements of the portion 122 areconfigured, aggregately, to form the irradiance pattern (such as, forexample, the pattern having a substantially sinusoidal cross-sectionalprofile) on the surface 114. The optical train also includes a receiving(or fringe-detecting portion) 134 optically coupled with the sourceportion 122 through the substrate 114 and structured to redirect lightreflected by the substrate 114 to the detector 120.

The spatial filter 128 of the source portion 122 of thefringe-projection sub-system is positioned to block the zerothdiffraction order of light distribution formed, in reflection, by thegrating 110 and let only the +1 and −1 orders pass towards the substrate114. The projection of fringes onto the plane of the detector 120represents, therefore, the interference of two off-axis plane waves(respectively corresponding to +1 and −1 orders of diffraction of lightat the grating 110) on the plane of the detector and, at the same time,the imaging of the sinusoidal grating 110 onto the detector 120 with aportion of the sub-system 100.

In further reference to FIG. 1, because the grating 110 is imaged ontothe surface 112 with a combination of optics forming an afocal opticalrelay, the fringe pattern 140 formed on the surface 112 at anywavelength generated by the source 124 has the same spatial frequency,while plane waves arriving at the surface 112 from the lens 126C atdifferent wavelengths are incident onto the surface atrespectively-corresponding different angles.

The irradiance fringe(s) re-imaged onto the plane of the detector 120with the use of reimaging optics 136A, 136B, 138 (and, optionally,additional optical elements, not shown) are used to determine theinitial or nominal position of the surface 112 and changes in positionof this surface with respect to the z-direction (defined as a directionalong a normal to the surface 112, that typically coincides with adirection of wafer exposure during the operation of the lithographicexposure tool). Such determinations are made with the use of aspecifically programmed data-processing electronic circuitry 150 thatmay include a computer processor. As schematically shown in FIG. 2, ifand when the surface 112 moves along the z-axis by Δz and assumes a newposition 112′ (or, alternatively, when the profile or local height ofthe surface 112 changes, which produces the same results), a fringe 210from the fringe pattern 140 is shifted laterally (from 210 to 210′) inthe same nominal plane by the distance Δx that is generally a functionof Δz and the angle θ of incidence of light onto the surface 112. Theoutput from the photodetector 120 (a position of which is defined by anoptical conjugate of the nominal position of the surface 112) is used todetermine the z-axis adjustments and corresponding focus adjustmentsrequired to keep the surface 112 at the optical focus of the system. Ina conventionally-used fringe-projection AF system, a portion of lightassociated with the sub-system 100 can be delivered to a referencemirror (for example, one disposed substantially parallel to and inproximity with the substrate 114) to form a reference signal. Additionaldetails of typical fringe projection AF system can be found in, forexample, commonly assigned U.S. Patent Application Publications2011/0071784 and 2012/0008150, the disclosure of each of which isincorporated herein by reference.

In further reference to FIG. 1, a typical fringe-projection sub-system100 is structured such that light at any wavelength from the source 124impinges on the same diffraction grating 110 at the same incident angle,and that the determination of the z-profile of the target surface 112 iscarried out based on the measurements performed at at least twodifferent wavelengths. Since the grating 110 is illuminated with light125 at different wavelengths, and since the grating 110 is generallyilluminated off-axis, the angle between the beams corresponding to +1and −1 diffraction orders formed at the grating 110 is wavelengthdependent, which causes a weak wavelength dependence of the measurementof the z-profile of the surface 112. For the sake of simplicity, suchdependence may be neglected.

The spatial shift of a fringe pattern 140 (in one case—a sinusoidaldistribution of irradiance) along the surface 112 (as compared to theposition of a fringe pattern on a nominally located surface 112) and acorresponding shift of the image of this fringe pattern on the plane ofthe detector 120 are caused not only by the change of profile (ormovement) of the surface 112 along the z-axis, but also by the change inindex of refraction of the medium (such as air) through which lightforming the fringe pattern arrives at the surface 112. (In practice,most of the optical elements of the receiving portion 134 of the systemcan be contained in a tube or barrel with a stable environment, therebyreducing practical implications of any residual errors attributed toair-movement in the portion 134 and making errors caused by the airnon-uniformity in the wafer space dominate.)

To this end, FIG. 3 provides a simplified illustration of changes in anoptical path of light traversing the fringe-projection sub-system, whichchanges are associated with air flow that is practically always presentduring the operation of the AF system of the invention. The typical airturbulences evolve slowly with time, and temporal frequencies associatedwith the related change of the optical path length(s) are typically onthe order of about several tens of Hz or less (for example, at about 20Hz or so). In FIG. 3, light portions at wavelengths λ₁, λ₂ in +1 and −1diffraction orders are shown, respectively, with lines 310, 312, 314,and 314 with respect to the local optical axis 320 of the overall lightbeam. When, due to some change in local conditions along the opticalpath associated with the fringe-projection system such as air turbulenceor other air-turbulence effects (ATE) and related experimental errors,the air temperature, for example, changes by ΔT, the correspondingoptical path difference (OPD_(ATE)) for the light portions correspondingto the +1 and −1 diffractive orders causes the lateral shift of fringesin the plane of the detector 120. Such ATE-dependent shift is normallyerroneously interpreted by the data-processing electronic circuitry 150of the AF system as a change in vertical (z-axis) profile or z-axisrepositioning of the surface 112, thereby contributing to the error indetermination of the required optical focus correction.

Generally, any portion of the path of light through thefringe-projection system where there is an optical path differencebetween the +1 and −1 orders arriving from the diffraction grating 110,may in practice cause such errors in practice. It should be noted thatthe OPD_(ATE)-based error is not reduced to zero if and when the thermaldistribution of air in the fringe-projection system does not change intime. When the distribution of temperature along the optical path oflight in the fringe-projection system is fixed in time, suchdistribution creates an offset (bias, or “ATE fingerprint” of thefringe-projection system) in phase, which may be spatially non-uniformand vary along x- and/or y-axis. This phase-bias that is added to thephase change of light contributed by profile/position of the wafer. Thedisclosed methodology provides for measurement of such bias or“fingerprint” phase, which is then subtracted from the measurement datato correct the measurement data for the presence of the ATE-relatederror.

It would be appreciated by a person of skill in the art that suchATE-dependent phase shift φ_(ATE) and the corresponding contribution tothe lateral shift of the fringe pattern caused by the ATE, the value ofwhich can be determined based on fringe-dependent irradiance dataacquired from the detector 120, is approximately reciprocally dependenton wavelength. In comparison, the phase shift φ_(Z) and thecorresponding contribution to the lateral shift of the fringe patterncaused by the change in z-profile of the measured surface 112 depend onthe angle θ of incidence of light at wavelength λ onto the surface 112,and the difference Δθ between the angular directions of propagation ofthe +1 and −1 orders of diffraction from the grating 110, Δθ=θ₊₁−θ⁻¹.(It follows from the grating equation that Δθ can be considered to varyapproximately linearly with wavelength.) The overall, aggregate estimateof the phase {tilde over (z)}. A derived from the measurements of thez-profile of the surface 112 with the AF system that employs afringe-projection sub-system can be expressed, therefore, as

{tilde over (z)}·A=φ _(Z)+φ_(ATE)=φ_(Z) +b·OPE _(ATE)  (1a),

or, in terms of geometrical distances,

{tilde over (z)}=z(x,y,λ)+(error term due to APE)  (1b)

where other error terms are neglected; where A [radian/μm] is thecoefficient of sensitivity of an AF system measurement response to thechanges in the actual topographic profile z=z(x,y) of the surface 112;and where b is a coefficient of sensitivity of the measurement to ATE,which is expressed in radians of a fringe-phase per micrometer of theOPD_(ATE) and which, therefore, is wavelength dependent.

As the profile z(x,y) of the surface 112 is measured, in practice, withthe AF system that employs the fringe-projection system operating at atleast two wavelengths λ₁, λ₂, an embodiment of the system of theinvention normally generates at least two respectively-correspondingempirical estimates {tilde over (z)}₁, {tilde over (z)}₂ of the changesin the surface 112 profile corresponding to the measurements at thesemultiple wavelengths:

{tilde over (z)} ₁ A ₁=φ_(Z1) +b ₁ OPD _(ATE)  (2a), and

{tilde over (z)} ₂ A ₂=φ_(Z2) +b ₂ OPD _(ATE)  (2b),

where φ_(Zi) =A _(i) ·z  (2c)

and the empirically measured phase terms {tilde over(z)}_(i)A_(i)=(λ_(i), θ_(i), T_(i)) are in radians.

The idea of the invention stems from the realization that when afringe-projection system is judiciously structured to ensure that (i)pair of the beams of light used for the measurement of the z-profile ofthe target surface 112—for example, the beams of light at λ₁ or λ₂—areemployed as reference beams (in which case the fringe-projection systemdoes not require a reference mirror); and that (ii) the sensitivitycoefficients A_(i), as well as the coefficients b_(i), are different forthe measurements performed at different wavelengths and angles ofincidence, the profile of the target surface can be derived to besufficiently free from the ATE-related errors. As a welcome corollary ofsuch approach, the AF system is now simplified and made devoid of areference mirror.

As a result of this realization, the fringe-projection sub-system of theAF system of an embodiment of the present invention is devised such thatthe two pairs of beams of light, used for the measurements of the targetsurface profile at λ₁, λ₂, produce two corresponding sets of irradiancedata respectively representing the two types of unknown phase shifts(φ_(Zi) and φ_(ATEi)) that are coupled through a system of two equationsand, therefore, can be directly determined based on the results of themeasurements only.

An embodiment of the method for deriving the target surface profile thatis substantially free from the ATE-related errors employs an algorithmdirected at solving the system of equations (2a), (2b), (2c) withrespect to the x-and-y dependent profile figure z=z(x,y). Solved for theproduct of b₁ OPE_(ATE), the above equations result in

$\begin{matrix}\begin{matrix}{{b_{1}{OPD}_{ATE}} = {b_{1}{{OPD}_{ATE}\left( {x,y,t} \right)}}} \\{= \frac{{\frac{A_{1}}{A_{2}}\left( {{\overset{\sim}{z}}_{2}A_{2}} \right)} - \left( {{\overset{\sim}{z}}_{1}A_{1}} \right)}{\frac{A_{1}b_{2}}{A_{2}b_{1}} - 1}}\end{matrix} & (3)\end{matrix}$

Thereafter, the spatial function z defining the topography of the targetsurface 112 can be determined based on the measurement data representingthe results of the measurement at λ₁ as, for example

$\begin{matrix}\begin{matrix}{z = \frac{\phi_{Z_{1}}}{A_{1}}} \\{= {\frac{1}{A_{1}}\left\lbrack {{{\overset{\sim}{z}}_{1}A_{1}} - {b_{1}{OPE}_{ATE}}} \right\rbrack}}\end{matrix} & (4)\end{matrix}$

or, alternatively and in a fashion similar to that of Eq. (4), based onthe measurement data representing the results of the measurement at λ₁.

Determination of Empirical Constants.

The values of A₁, A₂ are pre-defined by the system design as follows.Although the fringe pattern is imaged onto the substrate 114 with somemagnification coefficient, for the purposes of illustration it can beassumed that the imaging is performed with a magnification coefficientof 1 (such that angles at the grating 110 are the same as correspondingangles at the substrate 114). The phase terms representing the fringeshift caused by the changes of topography of the surface 112 areexpressed as

$\begin{matrix}{{\phi_{Zi} = {\frac{8\; {\pi \cdot \sin}\; {\theta_{i} \cdot \sin}\; \Delta \; \theta_{i}}{\lambda_{i}}z}},} & (5)\end{matrix}$

where θ_(i) is the average angle of incidence of light at λ_(i) in +1and −1 orders of diffraction onto the surface 112 and Δθ is the angulardeviation between the +1 and −1 orders of diffraction. Both angles,θ_(i) and Δθ are calculated based at least in part on the centerwavelength λ, the pitch T of the grating 110, and the angle of incidenceof light from the source 124 onto the grating 110. In a specific casecorresponding to the embodiment 100 of FIG. 1, where the spatial filter128 is designed to block the zeroth order of diffraction, the pitch ofthe fringe pattern 140 is half that of the grating (i.e., equal to T/2).Accordingly,

$\begin{matrix}{A_{i} = {\frac{8\; {\pi \cdot \sin}\; {\theta_{i} \cdot \sin}\; \Delta \; \theta_{i}}{\lambda_{i}}.}} & (6)\end{matrix}$

As understood by a person of skill in the art, the coefficients b_(i)are but the coefficients of conversion between the optical pathdifference and phase:

b _(i)=2π/λ_(i)  (7)

and, in Eq. (3), b₂/b₁=λ₁/λ₂.

Substituting the above-defined terms in Eq. (4) provides that thetopography of the target surface 112 can be expressed as a function ofthe two empirically measured phase terms {tilde over (z)}₁A₁, {tildeover (z)}₂A₂, two system-dependent constants A₁,A₂, and the a prioriknown wavelengths chosen for operation of the fringe-projection systemof the embodiment of the invention:

$\begin{matrix}{z = {\frac{1}{A_{1}}\left\lbrack {{{\overset{\sim}{z}}_{1}A_{1}} - {b_{1}\frac{{\frac{A_{1}}{A_{2}}\left( {{\overset{\sim}{z}}_{2}A_{2}} \right)} - \left( {{\overset{\sim}{z}}_{1}A_{1}} \right)}{\frac{A_{1}\lambda_{2}}{A_{2}\lambda_{1}} - 1}}} \right\rbrack}} & (8)\end{matrix}$

It is appreciated that light sources with broadband spectrum can beused, in which case the values of wavelengths λ₁, λ₂ are determined as,for example, as spectrally weighted, average values. While in practicethe measurement of the phase terms may have a noise component to them(as may, for example, the estimated of OPD_(ATE) term) based on thesystem parameters λ, T, θ_(i), the amplification or reduction of suchnoise level can be generally addressed on the level of fringe-projectionsystem design.

It is worth noting that, when averaged over space (x,y) and/or time (t),a more accurate estimated of the OPD_(ATE) can be obtained. Averagingover space is useful because the OPD_(ATE) does not vary significantlyon a small spatial scale. For example, spatial variations of a givenparameter across a span of a few millimeters (for example, 3 to 5millimeters) can be sufficiently accurately modeled with the use of asecond-order polynomial, with the spatial sampling on the order of 0.05mm to 1 mm. Averaging over time may be particularly efficient because inpractice the time-sampling of the ATE-dependent phase term can beeffectuated at a frame-rate of about 1 to 2 kHz or so while theATE-caused phase itself changes at a rate of several tens of Hz.Addition of complementary measurements at additional wavelengths canfurther improve the estimation process based on the use of linearalgebra techniques such as, for example, least square approach.

Representative Target Substrate Support Stage and High-Speed Scanning.

In reference to FIG. 4, providing a top view of the target substrate 114positioned on a supporting stage 410 as part of the fringe-projectingsubsystem, the supporting stage 410 includes two regions 420, 430adjacent to and on the opposite sides of the substrate 114 along thedirection of scanning of the substrate 114 (y-axis as shown). Each ofthe regions 420, 430 (outlined schematically with dashed lines) isreflective at λ₁, λ₂ and sufficiently flat (for example, the entiresurface flatness is better than a few tens of microns peak-to-valleysuch that it is easily captured within the dynamic range of the AFsystem), contains no surface patterns and/or layered optical structuressuch as coatings (and, therefore, introduces no Goos-Hanchen (GH) shiftin the beam of light incident onto such regions), and has approximatelythe same z-height as the top surface 112 of the substrate 114. Thesurface profile of the regions 420, 430 is judiciously chosen such thatinteraction of incident light with the regions 420, 430 is substantiallyindependent from the wavelength and polarization of the incident light.Elements 442, 444 signify auxiliary elements of the system that may bedisposed in the proximity of the substrate 114.

In the simplest case, the supporting stage 410 with the substrate 114 onit is scanned along the y-direction starting at the region 420 (referredto as pre-mapping), through the region occupied by the substrate 114(mapping), and towards to and finishing with the region 430(post-mapping) at speeds that allow the scanning process to be completedin a fraction of time on the scale of which the ATE are pronounced. Inone example, the scan is completed in under 0.25 second or, in aspecific case, in about 0.1 seconds. FIG. 5A provides an areal map ofthe aggregate z-profile of the measured surface determined based on themeasurement data. During such scan, the OPD_(ATE) data and thecorresponding error in estimation of the surface profile of thesubstrate 114 (referred to as the AT-signature) are measured across theregion 420 (along the line A of FIG. 5A) and across the region 430(along the line B of FIG. 5A) and plotted in FIG. 5B as “A, raw data”and “B, raw data” together with the corresponding polynomial fits “A,fit” and “B, fit”. Provided the relative slow rate of change of the airturbulence, the air turbulence effects can be reliably assumed to varylinearly across the measured field between the values corresponding tothe curves “A, fit” and “B, fit” of FIG. 5B. So linearly approximated,the OPD_(ATE)(x,y) is shown in the form of a 2D map in FIG. 5C. Bysubtracting the 2D map of FIG. C from that of FIG. 5A and addressingauxiliary sources of noise in data, the estimation of the 2D z-profileof the target surface 112 of the substrate 114 is made that issubstantially free from the contribution cause by the ATE-errors.

Mismatch of Optical Paths of Different Measuring Beams and Other Causesof Measurement Noise.

Referring again to FIG. 1, it is appreciated that in practice, as thetarget substrate 114 of FIG. 1 is imaged onto the detector 120 withminimal (if any) chromatic aberrations, the measurement beams atdifferent wavelengths λ₁, λ₂ overlap at the plane of the detector 120,and in the proximity of the target substrate, and at the targetsubstrate. However, these beams do not follow exactly the same opticalpath upon propagation through the fringe projection sub-system 100between the relaying optics 126C and 136A. Accordingly, the measurementbeams follow slightly different optical paths across the pocket of airnon-uniformity or turbulence shown as ΔT in FIG. 3. The diagram of FIG.6 illustrates this situation for the +1 orders of diffraction only (forthe simplicity of illustration the −1 order of diffraction has beenomitted), showing the minimum (if any) separation (marked as ΔZ˜0)between the measurement beams M₁ (λ₁, T, θ₁) and M₂ (λ₂, T, θ₂) withinthe region of interest defined by the measurement span 610 at thesubstrate 114, and substantial spatial separation (along the z-axis asshown and marked as ΔZ˜max) between these beams near the relayingoptics.

The quantification of the value of ΔZ is a function of the measurementspan 610, distance d from the center C of the measurement span 610 tothe lens 136A, and the angle α between the directions of propagation ofthe beams M₁ and M₂ at the lens 136A as ΔZ˜(d/2)*tan (α). A person ofskill in the art will appreciate, therefore, that for a given thresholdlevel of ATE-caused error in the target surface profile measurement, thehigher the spatial correlation of the regions or cells withair-turbulence that measurement beams traverse, the higher thedivergence between the measurement beams M₁, M₂ (and the larger theseparation ΔZ) that an embodiment of the fringe-projection sub-systemcan afford. The fringe-projection AF system of an embodiment of theinvention delivers practical results while tolerating a certain level oferrors the range of which is defined, at least in part, by the space ofparameters of the fringe-projection sub-system. Some of the examples arepresented below. It is worth mentioning that, while the pitch of thegrating 110 of the fringe-projection system can be used as ameasurement-error varying parameter, in practice any change in thegrating pitch is associated with the increased complexity of the opticalportion of the AF-system as it changes the mutual angular positioning ofthe AF-system components and is subject to strict practical spatialconstraints.

Example 1

One way of quantifying the operational space of an embodiment of theinvention is to define a dimensionless figure of merit or error factorEF representing, for example, a ratio of an estimate of the ATE-causedcontribution to the profile of the target surface to the overall figurerepresenting the profile of the target surface determined during themeasurement. EF=1 means that any random errors in the measurement suchas errors due to shot noise, for example, are not amplified, Eq. (8); avalue of EF>1 means these random errors are amplified.

A fringe-projection subsystem employs a grating 110 with a pitch T=678microns and is structured such that the averaged (over the +1 and −1orders of diffraction) angles of incidence of the measurement beams atλ₁, λ₂ onto the surface 112 of the substrate 114 areAOI1=AOI2=θ₁=θ₂=86°. For λ₁=0.42 μm and variable λ₂, the plots in FIG.7A illustrate dependencies of the EF and the separation distanceΔZ_(ave) (representing ΔZ averaged over d=100 mm). For the measurementutilizing λ₂=0.5 μm, for example, the EF is about 6 nm/nm, whileΔZ_(ave) is about 0.08 mm. In comparison, for the measurement utilizingλ₂=0.8 μm, EF is about 2 nm/nm, while ΔZ_(ave) is about 0.4 mm. It isrealized, therefore, that there is a practical trade-off between thedegree by which the ATE-caused error in the surface-profile measurementcan be reduced by choosing the operational wavelengths of the system ofthe invention and a mismatch of optical paths of the light beams atdifferent wavelengths propagating through the fringe-projectionsub-system.

Example 2

The plots of FIG. 7B schematically illustrate the influence of thevariation of the angle of incidence of the measurement beam (at λ₂) onthe target surface 112. Specifically, the plots are showing thedependence of the EF and ΔZ_(ave) on AOI2=θ₂ for the fringe-projectionsub-system of Example 1 in which the wavelength λ₂ is fixed at 0.68micron. Generally, increasing the angle of incidence leads to reductionof the error factor.

Example 3

Another example, the assessment of errors in which is shown in FIG. 7C,illustrates the measurement set-up according to that of Example 2, inwhich both of the measurement wavelengths are set to the same valueλ₁=λ₂=0.42 μm. Under these conditions, the EF of 2 nm/nm can be achievedat AOI2=θ₂=87.25 degrees with ΔZ_(ave) of about 0.2 mm. In general,however, the choice of spectrally-close measurement wavelengths is notnecessarily preferred as the rate of change of the EF and/or ΔZ_(ave)with respect to the change in the AOI2=θ₂ is higher than that for themeasurement utilizing the wavelengths with large spectral separation.

The error factor can be practically reduced by, for example, noticingthat the ATE-effects are developing at the time scale that is slow ascompared to the rate of measurement sampling and averaging the resultsof multiple measurements. For example, if the EF=2, the averaging can bemade over 4 measurement frames, as a result of which the EF value isreduced by sqrt(4)=2 to EF=1. In addition or alternatively, theconstraints imposed on numerical aperture (NA) of the imaging optics ofthe AF system should be taken into account. (Typical value of numericalaperture of the AF system is about NA˜0.02 at the angle of incidence, oflight onto the target surface 112, of about 86 degrees.) Additionalcauses of practical errors include the errors in determination of A₁,A₂, b₁, and b₂ of Eqs. (6, 7). The errors in determination of theseempirical parameters can be sufficiently reduced by accuratecharacterization of the wavelengths used for the measurements and thespectral filtering done by the optical system, which affects theaccuracy of the determination of b_(i). The values of A_(i) can besufficiently accurately calibrated based on z-position monitoring of theAF-system (which, in a typical lithography system is at the level ofsub-nm).

Example of Photolithographic (Exposure) Apparatus.

FIG. 8 provides a schematic view illustrating a photolithographyapparatus 800 (referred to interchangeably as scanner, imaging system,exposure apparatus, exposure tool, etc.) utilizing an embodiment of thefringe-projection system of the present invention. The wafer positioningunit 852 includes a wafer stage 851, the following stage 803A and anactuator 806. The wafer stage 851 comprises a wafer chuck that holds awafer W and an interferometer mirror IM. The exposure apparatus 800 canalso include an encoder to measure stage position (not shown forsimplicity of illustration). The base 801 may be supported by aplurality of isolators 854 (or a reaction frame), a least one of whichmay include a gimbal air bearing. The following stage base 803A issupported by a wafer stage frame (reaction frame) 866. The additionalactuator 806 is supported on the ground G through a reaction frame. Thewafer positioning stage 852 is structured so that it can move the waferstage 851 in multiple (e.g., three to six) degrees of freedom underprecision control by a drive control unit and system controller (notshown), and position and orient the wafer W as desired relative to theprojection optics 846. In the specific embodiment 800, the wafer stage851 may have six degrees of freedom and utilizes forces vectored in theZ direction and generated by the x-motor and the y-motor of the waferpositioning stage 852 to control a leveling of the wafer W. However, awafer table having three degrees of freedom (such as for example Z,θ_(X), θ_(Y)) or six degrees of freedom can be attached to the waferstage 851 to control the leveling of the wafer. The wafer table mayincludes the wafer chuck, at least three voice coil motors (not shown),and a bearing system. The wafer table may be levitated in the verticalplane by the voice coil motors and supported on the wafer stage 851 bythe bearing system so that the wafer table can move relative to thewafer stage 851.

The reaction force generated by the wafer stage 851 motion in the Xdirection can be canceled by motion of the base 801 and the additionalactuator 806. Further, the reaction force generated by the wafer stagemotion in the Y direction can be canceled by the motion of the followingstage base 1033A.

An illumination system 842 is supported by a frame 872. The illuminationsystem 842 projects radiant energy (e.g., light) through a mask patternon a reticle R, which is supported by and scanned using a reticle stage.Alternatively, in the case of systems using extreme ultraviolet (EUV)radiation, radiant energy is reflected by the reticle R. The reticlestage may have a reticle coarse stage for coarse motion and a reticlefine stage for fine motion. In this case, the reticle coarse stagecorresponds to the translation stage table 810, with one degree offreedom. The reaction force generated by the motion of the reticle stagecan be mechanically released to the ground through a reticle stage frameand the isolator 854 (in one example—in accordance with the structuresdescribed in JP Hei 8-330224 and U.S. Pat. No. 5,874,820, the entirecontents of each of which are incorporated by reference herein). Thelight is focused by a projection optical system (lens assembly) 846supported on a projection optics frame and released to the groundthrough isolator 854. The lens assembly 846 may include transmittingglass elements (refractive), reflecting mirrors (reflective) or acombination of the two (catadioptric) such as discussed, for example, inco-assigned patent application 61/907,747 and U.S. Pat. No. 8,705,170,the disclosure of each of which is incorporated herein by reference.

An interferometric system 856 (in one implementation—a fringe-projectionAF system according to an embodiment of the present invention) issupported on the projection optics frame and configured to detect theposition of the wafer stage 851 and generate output data representingthe position of the wafer stage 851 to the system controller 862. Asecond interferometer 858 can be supported on the projection opticsframe and configured t0 detect the position of the reticle stage andoutputs the information of the position to the system controller. Thesystem controller controls a drive control unit to position the reticleR at a desired position and orientation relative to the wafer W or theprojection optics 846.

There are numerous different types of photolithographic devices whichcan benefit from employing an embodiment of the present invention. Forexample, the apparatus 800 may comprise an exposure apparatus that canbe used as a scanning type photolithography system, which exposes thepattern from reticle R onto wafer W with reticle R and wafer W movingsynchronously. In a scanning type lithographic device, reticle R ismoved perpendicular to an optical axis of projection optics 846 byreticle stage and wafer W is moved perpendicular to an optical axis ofprojection optics 846 by wafer positioning stage 152. Scanning ofreticle R and wafer W occurs while reticle R and wafer W are movingsynchronously but in opposite directions along mutually parallel axesparallel to the x-axis.

Alternatively, the exposure apparatus 800 can be a step-and-repeat typephotolithography system that exposes reticle R, while reticle R andwafer W are stationary. In the step and repeat process, wafer W is in afixed position relative to reticle R and projection optics 846 duringthe exposure of an individual field. Subsequently, between consecutiveexposure steps, wafer W is consecutively moved by wafer positioningstage 852 perpendicular to the optical axis of projection optics 846 sothat the next field of semiconductor wafer W is brought into positionrelative to projection optics 846 and reticle R for exposure. Followingthis process, the images on reticle R are sequentially exposed onto thefields of wafer W so that the next field of semiconductor wafer W isbrought into position relative to projection optics 846 and reticle R.

However, the use of the exposure apparatus 800 schematically presentedin FIG. 8 is generally not limited to a photolithography system forsemiconductor manufacturing. The apparatus 800 (an exposure apparatus),for example can be used as an LCD photolithography system that exposes aliquid crystal display device pattern onto a rectangular glass plate ora photolithography system for manufacturing a thin film magnetic head.

In the illumination system 842, the illumination source can be a sourceconfigured to generate light at g-line (436 nm), i-line (365 nm), or toinclude a KrF excimer laser (248 nm), ArF excimer laser (193 nm), F₂laser (157 nm) or to generate radiation in EUV (for example, at about13.5 nm).

With respect to projection optics 846, when far ultra-violet rays suchas the excimer laser is used, glass materials such as quartz andfluorite that transmit far ultra-violet rays are preferably used. Whenthe F₂ type laser, projection optics 846 should preferably be eithercatadioptric or refractive (a reticle should also preferably be areflective type). When extreme ultra-violet (EUV) rays or x-rays areused the projection optics 46 should preferably be fully reflective, asshould the reticle.

With an exposure device that employs vacuum ultra-violet radiation (VUV)of wavelength 200 nm or shorter, use of the catadioptric type opticalsystem can be considered. Examples of the catadioptric type of opticalsystem include the disclosure Japan Patent Application Disclosure No.8-171054 published in the Official Gazette for Laid-Open PatentApplications and its counterpart U.S. Pat. No. 5,668,672, as well asJapanese Patent Application Disclosure No. 10-20195 and its counterpartU.S. Pat. No. 5,835,275. In these cases, the reflecting optical devicecan be a catadioptric optical system incorporating a beam splitter andconcave mirror. Japanese Patent Application Disclosure No. 8-334695published in the Official Gazette for Laid-Open Patent Applications andits counterpart U.S. Pat. No. 5,689,377 as well as Japanese PatentApplication Disclosure No. 10-3039 and its counterpart U.S. Pat. No.5,892,117 also use a reflecting-refracting type of optical systemincorporating a concave minor, etc., but without a beam splitter, andcan also be employed with this invention. The disclosure of each of theabove-mentioned U.S. patents, as well as the Japanese patentapplications published in the Office Gazette for Laid-Open PatentApplications is incorporated herein by reference.

Further, in photolithography systems, when linear motors that differfrom the motors shown in the above embodiments (see U.S. Pat. No.5,623,853 or U.S. Pat. No. 5,528,118 for example) are used in one of awafer stage or a reticle stage, the linear motors can be either an airlevitation type employing air bearings or a magnetic levitation typeusing Lorentz force or reactance force. Additionally, the stage couldmove along a guide, or it could be a guideless type stage that uses noguide. The disclosure of each of U.S. Pat. Nos. 5,623,853 and 5,528,118is incorporated herein by reference.

Alternatively, one of the stages could be driven by a planar motor,which drives the stage by electromagnetic force generated by a magnetunit having two-dimensionally arranged magnets and an armature coil unithaving two-dimensionally arranged coils in facing positions. With thistype of driving system, either one of the magnet unit and the armaturecoil unit is connected to the stage, and the other unit is mounted onthe moving plane side of the stage.

Movement of the stages as described above generates reaction forces thatcan affect performance of the photolithography system. Reaction forcesgenerated by the wafer (substrate) stage motion can be mechanicallyreleased to the floor (ground) by use of a frame member as described inU.S. Pat. No. 5,528,118 and published Japanese Patent ApplicationDisclosure No. 8-166475. Additionally, reaction forces generated by thereticle (mask) stage motion can be mechanically released to the floor(ground) by use of a frame member as described in U.S. Pat. No.5,874,820 and published Japanese Patent Application Disclosure No.8-330224. The disclosure of each of U.S. Pat. Nos. 5,528,118 and5,874,820 and Japanese Patent Application Disclosure No. 8-330224 isincorporated herein by reference.

Example of Environment of an Embodiment of the System

The present invention may be embodied in different forms such as asystem, method, or computer program product. For example, those skilledin the art should readily appreciate that functions, operations,decisions, etc. of all or a portion of a method of the invention may beimplemented as computer program instructions, software, hardware,firmware or combinations thereof. Those skilled in the art should alsoreadily appreciate that instructions or programs defining the functionsof the present invention may be delivered to a processor in many forms,including, but not limited to, information permanently stored onnon-writable storage media (e.g. read-only memory devices within acomputer, such as ROM, or devices readable by a computer I/O attachment,such as CD-ROM or DVD disks), information alterably stored on writablestorage media (for example, floppy disks, removable flash memory andhard drives) or information conveyed to a computer through communicationmedia, including wired or wireless computer networks.

In addition, while the invention may be embodied in software, thefunctions necessary to implement the invention may optionally oralternatively be embodied in part or in whole using firmware and/orhardware components (such as combinatorial logic, Application SpecificIntegrated Circuits or ASICs, Field-Programmable Gate Arrays or FPGAs,or other hardware or some combination of hardware, software and/orfirmware components), and may include an specific electronic circuitryor a processor controlled by instructions stored in a tangible,non-transient memory medium. The computer-usable or computer-readablemedium may be, for example, an electronic, magnetic, optical,electromagnetic, infrared, or semiconductor system, apparatus, device,or propagation medium. For example, computer-usable or computer-readablemedium may include a tangible non-transitory storage medium, such as,without limitation, a random access memory (RAM), a read-only memory(ROM), an erasable programmable read-only memory (EPROM or Flashmemory), a compact disc read-only memory (CDROM), and/or an opticalstorage memory medium, or any other memory, or combination thereof,suitable for storing control software or other instructions and data.The computer-usable or computer-readable medium may comprise and/or becomplemented with an apparatus that contains, stores, communicates,propagates, or transports program code for use by or in connection withthe instruction execution system, apparatus, or device. The computerprogram product may comprise program code stored in a computer readablemedium that, when executed on a computing device, causes the computingdevice to perform and/or govern one or more of the processes describedherein. The computer program product can be written in any conventionalprogramming language (such as, in one example, C++) or the like.

FIG. 9 provides an illustration of environment 910 for managing theprocesses in accordance with the invention. The environment 910 includesa server 912 that can perform the processes described herein usingappropriately structured computer program code(s). As should beappreciated by those of skill in the art, the server 912 includes acomputing device 914 having one or more processors 920, memory 922, aninput-output (I/O) interface 924, and a bus 926. The memory 922 caninclude local memory employed during actual execution of programcode(s), as one non-limiting example. The server 912 and/or computingdevice 914 are configured to read and/or receive information from thescanner 800, and use this information to predict critical dimensions ofthe images pattern and placement of its edges across the wafer that isbeing processed in the scanner 800. As used herein, the terms scannerand scanner apparatus refer to a photolithography apparatus (e.g.,imaging system, exposure apparatus, etc.) used in lithography.

The one or more processors 920 may be dedicated processors programmedfor execution of particular processes or combination of processes inaccordance with the invention, which may be performed on the server 912and/or the computing device 914. The server 912 and/or computing device914 may also be dedicated to particular processes or combination ofprocesses in accordance with the invention. Accordingly, the computingdevice 914 and/or server 912 can include any combination of generaland/or specific purpose hardware (e.g., one or more electronic circuitssuch as dedicated processors 920) and/or computer program code(s). Theserver 912 and/or computing device 914 are configured to communicateover any type of communications link, such as, for example: wired and/orwireless links; any combination of one or more types of networks (e.g.,the Internet, a wide area network, a local area network, a virtualprivate network, etc.); and/or utilize any combination of transmissiontechniques and protocols.

The computing device also includes an I/O device 928 that may beexternal to either the computing device 914 or the server 912. The I/Odevice 928 can be, for example, a device that is configured to enable anindividual (user) to interact with the computing device 914, such as adisplay equipped with GUI. In embodiments, the user can enterinformation into the system by way of the GUI (I/O device 928). In oneexample, the input items can be accessible to the user by a dialog box.In addition, it is contemplated that the I/O device 928 is configured tolead the user through the input requirements by providing input boxesfor textual input or pointer action.

By way of illustration, the I/O device 928 is configured to accept dataassociated with the scanner 800 and reticle/mask R of FIG. 1, amongstother information. The scanner data can include, for example,user-defined laser wavelength, laser bandwidth, laser spectrum,immersion and dry exposure data, a default index of refraction (forwater), pupil intensity, immersion exposure, threshold information(e.g., low intensity information from pupilgram files), polarizationinformation, etc. The mask information may include, for example, editingcapabilities for amplitude and phase information, etc., as well asaccepting GDS or OASIS mask files].

The server 912 (and/or computing device 914) includes a centralizeddevice repository, e.g., tangible, non-transitory storage memory system930. In embodiments, the centralized device repository 930 is configuredand/or designed to store the computer code and library information(data).

The following notes are in order. References made throughout thisspecification to “one embodiment,” “an embodiment,” “a relatedembodiment,” or similar language mean that a particular feature,structure, or characteristic described in connection with the referredto “embodiment” is included in at least one embodiment of the presentinvention. Thus, appearances of these phrases and terms may, but do notnecessarily, refer to the same implementation. It is to be understoodthat no portion of disclosure, taken on its own and in possibleconnection with a figure, is intended to provide a complete descriptionof all features of the invention.

In addition, the following disclosure may describe features of theinvention with reference to corresponding drawings, in which likenumbers represent the same or similar elements wherever possible. It isunderstood that in the drawings, the depicted structural elements aregenerally not to scale, and certain components may be enlarged relativeto the other components for purposes of emphasis and clarity ofunderstanding. It is also to be understood that no single drawing isintended to support a complete description of all features of theinvention. In other words, a given drawing is generally descriptive ofonly some, and generally not all, features of the invention. A givendrawing and an associated portion of the disclosure containing adescription referencing such drawing do not, generally, contain allelements of a particular view or all features that can be presented isthis view, for purposes of simplifying the given drawing and discussion,and to direct the discussion to particular elements that are featured inthis drawing. A skilled artisan will recognize that the invention maypossibly be practiced without one or more of the specific features,elements, components, structures, details, or characteristics, or withthe use of other methods, components, materials, and so forth.Therefore, although a particular detail of an embodiment of theinvention may not be necessarily shown in each and every drawingdescribing such embodiment, the presence of this detail in the drawingmay be implied unless the context of the description requires otherwise.In other instances, well known structures, details, materials, oroperations may be not shown in a given drawing or described in detail toavoid obscuring aspects of an embodiment of the invention that are beingdiscussed. Furthermore, the described single features, structures, orcharacteristics of the invention may be combined in any suitable mannerin one or more further embodiments.

The invention as recited in claims appended to this disclosure isintended to be assessed in light of the disclosure as a whole, includingfeatures disclosed in prior art to which reference is made.

While the description of the invention is presented through the aboveexamples of embodiments, those of ordinary skill in the art understandthat modifications to, and variations of, the illustrated embodimentsmay be made without departing from the inventive concepts disclosedherein.

For example, an implementation of a method of the invention may include,in reference to FIG. 10: the acquisition, with an AF-system employing afringe-projection system devoid of a reference mirror, of phase-datarepresenting a profile of the target surface, at step 1010, and thedetermination of parameters representing sensitivities of theacquisition process to changes in optical path(s) of the measurementbeam(s) and/or the changes in the profile of the target surface itself,as step 1020. Such determination may be effectuated with a specificallyprogrammed computer processor, be made at least in part based on theopto-geometrical parameters of the measurement system, and be based onlyon data derived from light that has interacted with the target surface(in other words, not involve data that is not derived from light thathas interacted with the target surface). Thereafter, at step 1030,contributions of air-related effects to the measurement(s) of the targetsurface profile are calculated based on the acquired phase-data anddetermined sensitivity values. These contributions are then subtractedfrom the results of the surface profile measurements at step 1040,thereby removing the empirically-defined error attributed to ATE fromthe measurement results. The order and labeled steps of the logicalflow-chart of FIG. 10 are indicative of one embodiment of the presentedmethod. Other steps and methods may be conceived that are equivalent infunction, logic, or effect to one or more steps, or portions thereof, ofthe illustrated method.

It is understood, therefore, that the present invention is directed to asystem that includes a profile-generating element configured to generatea profile of a target surface from optical data derived from two or moreselected optical parameters associated with an irradiance pattern formedon the target surface, which optical parameters are selected to beindicative of the ambient air turbulence around the target surface sothat profiling errors induced in the generated target surface profile bythe ambient air turbulence are at least partially reduced. In anon-exclusive implementation, the profile generating element can includedata-processing electronic circuitry (for example, a processor) governedby program code embedded in a non-transitory tangible computer readablemedium. One specific non-exclusive implementation of the system willcomprise, in such case, a programmable data-processing circuitry; and atangible, non-transitory storage medium with program code thereon, whichcode, when used to program the programmable data-processing circuitry,enables said circuitry to generate a profile of a target surface basedon optical data representing only two or more optical parametersassociated with an irradiance pattern formed on the target surface, theprofile being substantially devoid of errors induced by ambient airturbulence around the target surface.

In addition or alternatively, the system may include tangiblenon-transitory computer-readable medium having a computer programtherein, the computer program including a program code which causes theprofile-generating element to receive optical data representing theirradiance pattern formed on the target surface; determine two or moreoptical parameters indicative of the ambient air non-uniformity inproximity to and/or around the target surface; and to determine theprofile of the target surface based on such determined opticalparameters, where the errors contributed to the determined profile bythe air non-uniformity are at least partially reduced.

Disclosed aspects, or portions of these aspects, may be combined in waysnot listed above. In view of the numerous possible embodiments to whichthe principles of the disclosed invention may be applied, the inventionshould not be viewed as being limited to the disclosed example.

What is claimed is:
 1. An autofocus system comprising: aprofile-generating element configured to generate a profile of a targetsurface from optical data derived from two or more selected opticalparameters representing an irradiance pattern that is formed on thetarget surface by a first optical portion of said autofocus system andacquired by an optical detection unit of the autofocus system, theoptical parameters selected to be indicative of non-uniformity ofambient air around the target surface such that a profiling error,induced in a measurement of said profile by said non-uniformity ofambient air, is at least partially reduced.
 2. An autofocus systemaccording to claim 1, wherein said first optical portion of saidautofocus system includes a diffraction grating configured as a beamsplitter that defines first and second beams from light incidentthereon, said first and second beams respectively containing light fromdifferent diffraction orders formed by said diffraction grating.
 3. Anautofocus system according to claim 1, configured to make first andsecond beams interfere at the target surface and further devoid of areference reflector interacting with light in any of said first andsecond beams.
 4. An autofocus system according to claim 1, wherein saidirradiance pattern is formed by diffraction orders that originatedwithin the first optical portion of said autofocus system from lighttraversing said first optical portion, said diffraction orders havingopposite signs.
 5. An autofocus system according to claim 1, wherein theprofile-generating element includes a computer device configured todetermine said profiling error based on multispectral data, said dataderived from images, of said irradiance pattern, formed at an opticaldetector of the optical detection unit with multiple beams of lightafter interaction with the target surface, said multiple beams of lightcharacterized by different parameters of said measurement.
 6. Anautofocus system according to claim 5, wherein said different parametersinclude a pitch of a diffraction grating of said autofocus system, saidgrating configured to generate different diffraction orders forming saidirradiance pattern on the target surface; a wavelength of lightcharacterizing a diffraction order generated at the diffraction grating;and a mean angle of incidence, on the target surface, of light deliveredfrom the first optical portion.
 7. An autofocus system according toclaim 6, wherein said mean angle of incidence is calculated based onangles of incidence, on said target surface, of said diffraction orders.8. An autofocus system according to claim 6, wherein said differentdiffraction orders have opposite signs.
 9. An autofocus system accordingto claim 5, wherein opto-geometrical parameters characterizing saidautofocus system include first figures of merit associated withsensitivity of the autofocus system to a change in said profile andcorresponding to said different parameters of said measurement, andsecond figures of merit associated with sensitivity of operation of theautofocus system to said non-uniformity of ambient air.
 10. Alithographic exposure system including an autofocus system according toclaim
 1. 11. An autofocus system comprising: an optical systemstructured to project an irradiance pattern containing interferencefringes onto a target surface and form an image of said irradiancepattern on an optical detector, said optical system characterized byopto-geometrical parameters; and electronic circuitry operably connectedwith the optical detector and configured to generate data representing aprofile of said target surface measured with said autofocus system, saidprofile having at least partially reduced errors, wherein said errorsare (i) associated with a difference between first and second opticalpaths defined by the optical system and the target surface and (ii)caused by non-uniformity of air in at least one of said first and secondoptical paths, wherein said errors are calculated based only on saidopto-electronic parameters and data representing said irradiancepattern.
 12. An autofocus system according to claim 11, wherein saidoptical system is structured to form said irradiance pattern byprojecting two plane waves onto the target surface.
 13. An autofocussystem according to claim 12, said optical system including adiffraction grating configured to form diffraction orders of oppositesigns from light incident thereon, and first optical subsystem of unitmagnification configured to form said irradiance pattern on the targetsurface by projecting said diffraction orders onto the target surface,said target surface and said diffraction grating being opticalconjugates of each other.
 14. An autofocus system according to claim 13,further comprising second optical sub-system of unit magnificationdisposed between said first optical elements and the optical detectorsuch that the target surface and said optical detector are opticalconjugates of each other. An autofocus system according to claim 11,wherein each of the first and second optical paths includes an area onthe target surface, and further devoid of a reference reflectorinteracting with light propagating along any of the first and secondoptical paths.
 15. An autofocus system according to claim 11, whereinthe electronic circuitry is configured to determine a measurement error,of said profile, that is caused by said non-uniformity of air, based onmultispectral data derived from images, said images formed at theoptical detector by multiple beams of light that (i) are formed withinsaid optical projection system from light propagating therethrough andthat (ii) have interacted with the target surface, different beams fromsaid multiple beams of light characterized by different parameters of ameasurement of said profile.
 16. An autofocus system according to claim15, wherein said parameters include a pitch of a diffraction grating ofsaid autofocus system, said grating configured to generate diffractionorders of different signs forming said irradiance pattern on the targetsurface; a wavelength of light characterizing a diffraction ordergenerated at the diffraction grating; and a mean angle of incidence, onthe target surface, of light delivered from the first optical portion.17. An autofocus system according to claim 15, wherein saidopto-geometrical parameters include first figures of merit associatedwith sensitivity of the autofocus system to a change in said profile andcorresponding to said different measurement wavelengths, and secondfigures of merit associated with sensitivity of operation of theautofocus system to said non-uniformity of air. A lithographic exposuresystem including an autofocus system according to claim
 11. 18. Anautofocus system comprising: an optical projection system containing adiffraction grating and structured to define interference fringes on atarget surface by projecting first and second beams on said targetsurface, the first and second beams formed by said diffraction gratingfrom light incident thereon; and a detection system containing anoptical detector and electronic circuitry and configured a) to receivefirst and second images of said interference fringes at the opticaldetector, the first image being formed at a first wavelength, the secondimage being formed at a second wavelength, the first and secondwavelengths being different, and b) to derive, based on directcharacterization of parameters of received first and second images withsaid electronic circuitry, phase data that are associated with anoptical path variation caused by non-uniformity of air in said opticalprojection system and that is measured between said first and secondbeams.
 19. An autofocus system according to claim 18, that does notcontain a reference reflector configured to interact with light from atleast one of said first and second beams.
 20. An autofocus systemaccording to claim 18, wherein the electronic circuitry includes aprocessor programmed to calculate a profile of said target surface andcorrect this profile by calculating an error of measurement, of saidprofile, caused by said non-uniformity of air.
 21. A lithographicexposure system including an autofocus system according to claim 18.